Since fiber reinforced plastics comprising a large number of reinforcing fibers and a matrix resin are high in specific strength and specific modulus, excellent in mechanical characteristics and have high performances such as weather resistance and chemical resistance, they are drawing attention also in industrial application and their demand is increasing year by year.
As a molding method of fiber reinforced plastic having high performance, autoclave molding method is mentioned in which material bases of semi-cured state (prepreg) of a large number of continuous reinforcing fibers impregnated with a matrix resin are laminated, and a fiber reinforced plastic is molded by curing the matrix resin by heat-pressurizing the obtained laminate in a high temperature-high pressure oven. This autoclave molding method is widely used.
On the other hand, as another molding method of fiber reinforced plastic, RTM (resin transfer molding) molding method is mentioned in which, for purpose of increasing production efficiency, material base comprising a large number of continuous reinforcing fibers (preform) shaped into a desired member shape is impregnated beforehand with a matrix resin, and the impregnated matrix resin is cured.
The fiber reinforced plastic obtained by these molding methods, since the large number of reinforcing fibers are continuous fibers, has excellent mechanical property. Also, since these large number of continuous fibers are arranged regularly in the material base, it is possible to design such that the fiber reinforced plastic has a necessary mechanical property by arrangement of the material bases to be laminated, and dispersion of mechanical property of the obtained fiber reinforced plastic is also small. However, on the other hand, since each of the large number of reinforcing fibers is a continuous fiber, it is difficult to form into a complicated shape such as a three dimensional shape, and these molding methods are used for production only of members of approximately flat shape.
As molding method suitable for molding a molded article having complicated shape such as a three dimensional shape, molding methods in which an SMC (sheet molding compound) sheet or a stampable sheet is used are mentioned.
The SMC molding method is a molding method in which an SMC sheet, made into semi-cured state by impregnating chopped fiber bundle, comprising a large number of reinforcing fibers which are usually cut into approximately 25 mm, with a thermosetting resin as matrix resin, is molded by heat-pressing by using a heat-press machine.
The stampable sheet molding method is a molding method in which a stampable sheet, made by impregnating chopped fiber bundle, comprising a large number of reinforcing fibers which are usually cut into approximately 25 mm, with a thermoplastic resin as matrix resin, is molded by being heated to melting point or more of the thermoplastic resin and arranging on a mold controlled to a predetermined temperature to be cold-pressed.
In many cases, molding is carried out by arranging an SMC sheet or a stampable sheet cut smaller than desired molded article shape on a mold and by extending (by flowing) the sheet arranged on the mold into the shape of the above-mentioned molded article by pressurizing. For that reason, by the flow of the matrix resin and a large number of the cut reinforcing fibers, a mold following capability into complicated shape such as a three dimensional shape can be obtained.
However, chopped fiber bundle used in the conventional SMC sheet or stampable sheet is produced by cutting a continuous reinforcing fiber bundle comprising a large number of unidirectionally arranged continuous reinforcing fibers in perpendicular direction to the arranging direction of reinforcing fiber. When a force is loaded on the molded article molded with a large number of such chopped fiber bundles and the matrix resin (fiber reinforced plastic), almost all of its force is born by a large number of chopped fiber bundles. That is, the large number of chopped fiber bundles shares the force with each other via the matrix resin.
In this case, in a conventional molded article, since the end of each reinforcing fiber at tip portion of the chopped fiber bundle is located at a plane perpendicular to the arranging direction of reinforcing fiber, a state in which the force is immediately shared to an adjacent chopped fiber bundle is arisen. In this state, a stress concentration is generated at the portion. When this stress concentration is generated, even in the case where the force is small, the portion is broken by the stress concentration, to generate a crack in the molded article. The generated crack is linked to other cracks, to bring about a breakage of the whole molded article.
In the conventional SMC sheet or stampable sheet, since length of each reinforcing fiber of the chopped fiber bundle used therein is about 25 mm, although a flowability of the reinforcing fiber at forming a molded article can be obtained, since the end of each reinforcing fiber is located at the plane perpendicular to the arranging direction of reinforcing fiber, there was a problem that a crack is easy to be generated in the molded article which is molded.
To solve this problem, some SMC sheets have been proposed (e.g., JP 62-048730 A and JP 01-163218 A) which are produced by using chopped fiber bundles consisting of a decreased number of reinforcing fibers to increase the entanglement of the chopped fiber bundles for increased densification which serves to prevent the generation and propagation of cracks in the molded material. On the other hand, in the thin chopped fiber bundle of which number of reinforcing fibers is decreased, since the chopped fiber bundle apt to cohere with each other, a dispersion in distribution or orientation of the reinforcing fibers in the molded article is easy to be generated, there is a problem that mechanical characteristics of the obtained molded article lowers.
It could therefore be helpful in view of these problems to provide a chopped fiber bundle having good flowability and mold conformability in the case where it is used as a molding material, and exhibits excellent mechanical property in the case where it is used as a fiber reinforced plastic, and a production method thereof.
It could also be helpful to provide a molding material and a fiber reinforced plastic in which the chopped fiber bundle is used, and a production method thereof.